This is from “The Methodology of Scientific Research Programmes,” in Criticism and the Growth of Knowledge, Lakatos & Musgrave (eds.) (1970) (full text), pp. 135-6.
I’ve added paragraph breaks and emphasis is mine
“Newton first worked out his programme for a planetary system with a fixed point-like sun and one single point-like planet. It was in this model that he derived his inverse square law for Kepler’s ellipse.
“But this model was forbidden by Newton’s own third law of dynamics, therefore the model had to be replaced by one in which both sun and planet revolved round their common centre of gravity. This change was not motivated by any observation (the data did not suggest an ‘anomaly’ here) but by a theoretical difficulty in developing the programme.
“Then he worked out the programme for more planets as if there were only heliocentric but no interplanetary forces.
“Then he worked out the case where the sun and planets were not mass-points but mass-balls. […] This change involved considerable mathematical difficulties, held up Newton’s work—and delayed the publication of the Principia by more than a decade.
“Having solved this ‘puzzle’, he started work on spinning balls and their wobbles. Then he admitted interplanetary forces and started work on perturbations. At this point he started to look more anxiously at the facts. Many of them were beautifully explained (qualitatively) by this model, many were not.[…]”
Notice that Newton is (per Lakatos) progressively refining his theory in a process of successively more challenging thought experiments.